Caused by an explosive increase in transmission capacity due to recent prevalence of the Internet, a large capacity increase and high speeding in optical fiber transmission are demanding. At present, an introduction of a system having a transmission speed of 10 Gbps is going forward in a backbone network, but for meeting a demand for further larger capacity, a transmitting system having a higher transmission speed is developing. For introducing a system having a transmission speed of more than 10 Gbps, it is important to further lower costs as compared to the system of 10 Gbps. Particularly in a signal source, miniaturizing an element and driving in low power are needed, and it is also necessary to lengthen a distance which can be transmitted without relay.
As a signal source widely used at present, a system of directly modulating a laser and a system of externally modulating CW light outputted from a laser are exemplified. In the direct modulation system, a current modulating drive of a distributed feedback laser (DFB-LD) is generally used. In the external modulation system, modulators can be classified into two types of modulators composed of an electroabsorption modulator and a modulator of a phase modulation type.
In the electroabsorption type, intensity modulation is performed using an absorption change due to electric field applied to a semiconductor waveguide. In a case of a quantum well structure, a quantum-confined Stark effect (QCSE) is generally used, and in a case of a bulk structure, a Franz-Keldysh effect is generally used. In this structure, an electro-absorption modulator integrated type (EA-) DFB-LD is widely used in this structure because of easy integration with DFB-LD.
As the phase modulation type, a Mach-Zehnder modulator is used. This modulator branches input light into two optical paths by a coupler and thereafter, generates a phase difference between the respective branch lights of the two optical paths by a change of a refractive index due to electric field impression. At the time of multiplexing two branch lights having the phase difference therebetween, the intensity change by the phase difference is used to perform the intensity modulation. In regard to the refractive index change, an electro-optic (EO) effect is used in addition to the QCSE effect and the Franz-Keldysh effect. LiNbO3 (LN) is widely and generally used as a material of the modulator.
The direct modulation system has a feature that the control is simple. However, there exists a problem that at current modulating, a frequency fluctuation in lasing wavelength of a laser is generated in accordance with intensity of a modulation signal. Since a propagation characteristic of an optical fiber as a transmission medium has frequency dispersion, an optical signal after the optical fiber propagation is degraded due to the frequency fluctuation of the optical signal.
FIG. 1 shows the construction of the conventional optical modulation signal transmitting system. An optical signal outputted from an optical modulation signal generating device 901 propagates through an optical fiber 902 and is received by an optical receiving device 903. It is assumed that an NRZ signal of 20 Gbps as shown in FIG. 2 is generated from a signal source of the optical modulation signal generating device 901. FIG. 3A shows an eye opening of an optical signal generated by directly modulating a laser light of the optical modulation signal generating device 901 with this NRZ signal. In the direct modulation system, a frequency fluctuation corresponding to the modulation signal is generated in the optical signal outputted from the laser. FIG. 3B shows an eye opening of an optical signal which in a case where a frequency fluctuation width is 10 GHz, propagates through the optical fiber 902 of 50 km and is received by the optical receiving device 903. It is found out that a waveform of the optical signal largely deteriorates through the optical fiber propagation.
For solving deterioration of this propagation characteristic, there is proposed a method in which a frequency filter is used to carry out mutual conversion between frequency amplitude and intensity amplitude for restricting the frequency fluctuation (for example, refer to Non-Patent Document 1). However, because of directly modulating the laser, there is a problem that an operation speed is limited by a relaxation oscillation frequency of the laser. At present, in a commercialized laser the relaxation oscillation frequency is the order of 10 GHz and it is difficult to realize modulation speed higher than that.
Since the electroabsorption modulator among the external modulation system performs an electric field drive without using a carrier, the high-speed modulation of 40 GHz is possible. However, in the modulator using the QCSE effect or the Franz-Keldysh effect, because of a drive in the vicinity of a band gap edge, a large refractive index change is generated in addition to a change in absorption coefficient. This causes a frequency fluctuation of an optical signal at the rising and falling of the optical signal. Since this frequency fluctuation causes deterioration of the optical signal at an optical fiber propagation time of the optical signal, it is difficult to perform long-distance transmission with no relay.
The phase modulation type modulator also has an advantage that likewise the high-speed modulation of 40 GHz is possible and the frequency fluctuation can be restricted. However, in a LN modulator most widely used, an element size is large and a drive voltage is higher as compared to that in a semiconductor. In addition, there is a problem that the LN modulator is not suitable for integration with DFB-LD.
The present invention is made in view of this problem and an object of the present invention is to provide an optical modulation signal generating device and an optical modulation signal generating method which realize high-speed modulation and long-distance transmission.
Patent Document 1: Japanese Patent No. 3832743 Non-Patent Document 1: P. A. Morton et al., “38.5 km error free transmission at 10 Gbit/s in standard fiber using a low chirp, specially filtered, directly modulated 1.55 mm DFB laser”, Electronics Letters, vol. 33, no. 4, pp. 310-311.